This is that riff that blues singers use. 
You sing something then play it. (many times) 
you can play it on almost any 2 strings, or play it  
in power chords. I think it sounds the best this way. 
 
|-----------| 
|-----------| 
|----2-0----| 
|--2-----2--| 
|-----------| 
|-----------| 
 
Here's a way to play it on a C harmonica. 
s - suck 
b - blow 
 
 5b 6s 6b 5b 
 
 
That's all folks! 
 
have fun!